Neural processor with holographic optical paths and nonlinear operating means

ABSTRACT

An optical apparatus for simulating a highly interconnected neural network is disclosed as including a spatial light modulator (SLM), an inputting device, a laser, a detecting device, and a page-oriented holographic component. The inputting device applies input signals to the SLM. The holographic component optically interconnects N 2  pixels defined on the spatial light modulator to N 2  pixels defined on a detecting surface of the detecting device. The interconnections are made by N 2  patterns of up to N 2  interconnection weight encoded beams projected by N 2  planar, or essentially two-dimensional, holograms arranged in a spatially localized array within the holographic component. The SLM modulates the encoded beams and directs them onto the detecting surface wherein a parameter of the beams is evaluated at each pixel thereof. The evaluated parameter is transformed according to a nonlinear threshold function to provide transformed signals which can be fed back to the SLM for further iterations.

This application is a continuation of U.S. application Ser. No.07/365,441, filed on Jun. 13, 1989, now U.S. Pat. No. 5,004,309, whichis a continuation-in-part of U.S. application Ser. No. 07/233,575, filedon Aug. 18, 1988, and now abandoned.

BACKGROUND OF THE INVENTION

The present invention pertains to an optical system including apage-oriented holographic means for providing a highly parallelcomputational device which simulates neural operation. Moreparticularly, the present invention pertains to a system includingholographic means comprising a plurality of essentially two-dimensional,spatially localized holograms arranged in an array for projectinginterconnection weight encoded beams to interconnect a spatial lightmodulator which modulates the beams in response to input signals and adetecting means which performs a nonlinear transformation on themodulated beams to produce output signals.

This invention relates to an optical system providing what has becomeknown in the art as a "neural network". Such a network comprises devicesthat simulate the responses of biological neurons. A model for a neuronN is shown in FIG. 1 to receive three inputs X₁, X₂, and X₃ at a devicewhich sums the inputs according to the simple equation S=X₁ +X₂ +X₃.Positive X's may be defined as excitatory and tend to make the modelneuron "fire", that is provide a nonzero output. Negative X's, definedas inhibitory, tend to prevent the model neuron from firing. A nonlinearoperator changes an output signal from the output S of the summingdevice into a new signal according to a particular response curve to bediscussed in detail infra. A low input signal to the nonlinear operator,that is a signal below some threshold, S₀, results in a zero output atthe nonlinear operator. A high input signal gives a fixed maximumoutput. An intermediate input results in an intermediate output. OutputS' from the nonlinear operator is applied to still other neurons aftermultiplication by a weighting factor W by a distributor. The signals W₁S', W₂ S', and W₃ S' are proportional to S' and may be either strong orweak excitatory signals or strong or weak inhibitory signals.

"Technological" or "artificial" neural networks also mimic biologicalneural networks by arrangement of the neurons in layers as indicated inFIG. 1. The information, memory, and problem solving methodscharacteristic of the system are determined by the interconnections inthe system, that is what is interconnected to what and with whatstrength. In providing a model neutral network, the model neurons aremade to communicate laterally on the same layer and to communicate withneurons in other layers.

For various applications, the power of systems simulating neuralactivity over conventional, sequential machines has been well recognizedby the prior art. For instance, U.S. Pat. No. 4,660,166 to Hopfieldstates that many practical problems take such an enormous amount ofcomputation that a solution in real time is not feasible. The Hopfieldpatent goes on to describe a network which electronically simulatesneural activity to provide a system capable of retrieving particularinformation from a memory in the system, in response to an interrogationof the system. The patentee describes such a retrieval system as anassociative memory, that is a memory that provides an output which is insome way associated with a particular input applied to the system. Suchan network comprises amplifiers characterized by nonlinear, continuousand sigmoidal response curves. The input is processed in parallel. Suchnetworks thus electronically process plural input signals to obtaincollective decisional responses to which all of the input signals make acontribution in the range from 0 to 100%.

U.S. Pat. No. 4,752,906 likewise relates to a system employing neuralcomputation to develop sequences of output vectors. This patentdescribes a neural network as having a highly parallel computationalcircuit comprising a plurality of electronic amplifiers. Each of theamplifiers feeds back its output signal to itself and to all of theother amplifiers.

Electronic implementations are inherently limited in the number ofinterconnections that can be made. It appears unlikely that anelectronic circuit providing for more than about 1,000,000 i.e. 1×10⁶interconnections is feasible. To attempt to attain such a large numberof interconnections in an electronic system results in very significantcross-talk problems. Further, electronic systems are seriously limitedby power requirements. Such systems are also limited by volume andweight requirements.

The development of optical systems to carry out computations hasprogressively advanced. In an article by H. J. Caufield, J. A. Neff andW. T. Rhodes, "Optical Computing: The Coming Revolution in OpticalSignal Processing, Laser Focus/Electro-Optics, November 1983, p. 100,earlier progress in the application of optics to mathematical operationsis reviewed. Examples of optical apparatus for performing digital matrixmultiplication are disclosed in U.S. Pat. Nos. 4,567,569 and 4,809,204.

Further, optical machines that demonstrate different approaches toassociative memory have been developed. One approach, developed by theCalifornia Institute of Technology, is referred to as photorefractivehologram neural networks. According to this approach, the selectivity ofthick holograms in photorefractive materials is used as the primarydriver for an optical associative memory. Systems based on this approachhave demonstrated significant ability to learn new data. A secondapproach is represented by U.S. Pat. Nos. 4,739,496 and 4,750,153. Inthe optical systems disclosed in these patents, multiple,high-resolution images are stored in a holographic medium. Wheninterrogated by an input image, the systems of the two latter patentsrecall the closest, most correct image stored. Even if these systems areaddressed with an incomplete version of one of the stored images, theywill output the complete image.

Limitations in the number of interconnections that can be made alreadyhave been recognized in optical systems designed according to thesefirst two approaches. The photorefractive holograms used in systemsaccording to the first and second approaches are known in the art asvolume holograms which have relatively large thicknesses. Due to thethickness of the holograms, intermodulation noise becomes an increasingfactor as the number of interconnections approaches 10¹⁰. Up to now,volume hologram approaches therefore have been limited to less than 10¹⁰interconnections. When this number of interconnections is approached orexceeded in a system relying upon volume holography, the performance ofsuch system lessens due to increasing problems in the way of lesseningdynamic range, increasing intermodulation noise and degeneracy in theinterconnections due to multiple order production by each modulationfrequency recorded.

SUMMARY OF THE INVENTION

The system of the present invention overcomes the problems inherent involume holograms and provides a neural network attaining 10¹²interconnections without the losses in performance experienced withprior art optical systems. The disclosed system according to the presentinvention can be implemented solely from optical components. It utilizesa page-oriented holographic means comprising an N×N arrays of very thinholograms arranged in a spatially localized or side by side manner on asingle holographic plate or substrate. In the preferred embodiments ofthe system of the invention, N is a large number which is contemplatedto equal or exceed 10³. Each individual hologram is formed to project apattern of N² individual light beams each onto its own selected pixel ofa spatial light modulator. When illuminated by laser light, thecomposite N×N holographic array therefore projects up to N⁴ individuallight beams onto N² resolvable areas or pixels on the spatial lightmodulator.

Input to the system is applied to the spatial light modulator. The inputis conveniently thought of as an input vector in the form of a vectorarranged in two dimensions and having N² elements. The lenses in thesystem direct each pattern of N² modulated beams projected onto each ofits N² pixels to a photodetector. Each pattern of encoded beams ismodulated according to a corresponding element of the input vector.

The light beams projected into the system of the present invention bythe holographic means thereby interconnect the input data applied to thespatial light modulator to associated pixels on the photodetector sothat each element of the input vector makes some contribution, in therange of from 0 to 100%, to each element of an output vector generatedby the detector. The projected beams are encoded by the holographicmeans with weights for each such interconnection made.

To complete simulation of the biological neural response, thephotodetector provides output signals which are nonlineartransformations of the signals applied to it from the spatial lightmodulator. In the preferred embodiment, the transformation performed bythe photodetector is described by a continuous, sigmoidal, monotonicallyincreasing function to define a nonlinear threshold response. Thetransformed output signals can be fed back to the spatial lightmodulator as input if desired or can be fed forward to a second spatiallight modulator.

The preferred embodiments of the system of the invention can be operatedas a bidirectional associative memory (BAM). When queried by an inputvector, X, these embodiments generate the closest paired vectors X_(out)and Y_(out) by performing a series of matrix vector multiplications andnonlinear transformations. Alternatively, the preferred embodiments canbe made to provide interconnections whereby the system simulatesoperation of a Hopfield neural network.

A processing system according to the present invention for simulating ahighly interconnected neural network comprises means for applying inputsignals, holographic means for providing optical signals indicative offirst predetermined interconnection weights, spatial light modulatingmeans responsive to the input signals for modulating the first opticalsignals to provide first modulated optical signals, means disposed in apath to detect the first modulated optical signals for providingdetection signals which are a nonlinear function of a parameter of theoptical signals, and means for directing the detection signals to thefirst spatial light modulating means as input signals.

Alternatively, the processing system for producing a highlyinterconnected neural network according to the present inventioncomprises a spatial light modulator which defines N² pixels, means forapplying input signals to the spatial light modulator, a laser, adetecting means and a page-oriented holographic means for opticallyinterconnecting the spatial light modulator and the detecting means. Theholographic means comprises an N×N array of spatially-localized planarholograms with each such hologram projecting a distribution of up to N²light beams onto an associated pixel of the spatial light modulator whenthe holographic means is illuminated by light from the laser. Theholograms encode a parameter of each of the projected light beams torepresent an interconnection weight. The spatial light modulatormodulates the projected light beams according to the input signals toprovide up to N⁴ modulated light beams. The detecting means is disposedin the path of the modulated light beams and comprises a detectingsurface defining N² pixels interconnected with associated pixels of thespatial light modulator so that each detecting surface pixel receives upto N² modulated light beams from its associated pixel of the spatiallight modulator, and means for nonlinearly transforming the parameter ofeach of the modulated beams to provide transformed signals.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects and features of the present invention will be even moreapparent from the following detailed description and drawings, and theappended claims. In the drawings:

FIG. 1 is a schematic view illustrating a model for an interconnectedneuron;

FIG. 2 is a ray diagram in block-diagrammic form of a preferredembodiment of the system according to the present invention;

FIG. 3 is a schematic plan view of a page-oriented holographic meanscomprising an N×N array of thin holograms incorporated in the system ofthe present invention;

FIG. 4 is a diagram useful in understanding the distribution of lightbeams on the light-receiving sides of the spatial light modulators inthe system of FIG. 2;

FIG. 5 is a diagram useful in understanding the distribution ofmodulated light beams directed from one of the spatial light modulatorsin the preferred system of FIG. 2 onto the other spatial lightmodulator;

FIG. 6 illustrates a sigmoidal, monotonic input-output responsecharacteristic utilized in the system of the present invention;

FIG. 7 is a block diagram of an exemplary multilayered or cascadingarrangement of optical systems according to the invention;

FIG. 8 is a diagram similar to FIG. 2 of an alternative embodiment ofthe system of the present invention;

FIG. 9 illustrates an alternative detector arrangement suitable for usein the preferred embodiments;

FIG. 10 illustrates a clipped input-output characteristic;

FIG. 11 is a diagram similar to FIG. 2 of another embodiment of thesystem of the present invention; and

FIG. 12 illustrates another nonlinear response input-outputcharacteristic suitable for use in the preferred embodiments of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION System Overview

In schematic form, FIG. 2 shows a preferred processing system 10 inaccordance with the present invention for simulating a highlyinterconnected neural network. All of the elements used in implementingpreferred system 10, as well as the other preferred embodiments, areconventional. According to the embodiment of FIG. 2, the processingsystem 10 is implemented with optical elements, however, as will beseen, the system could comprise various combinations of optical andelectrical components while remaining well within the scope of theinvention. Processing system 10 includes holographic means 11 providedby a first holographic array means 12 and a second holographic arraymeans 14. Each of holographic array means 12 and 14 is shown asilluminated by a source 16, 18, of laser light. Each holographic arraymeans 12 and 14 projects N⁴ individual beams to make the desired numberof interconnections in the system 10. Each individual light beam isencoded by its respective holographic array means 12, 14 to have one ormore parameters representing the particular weight accorded theinterconnection made by that beam. Each holographic array means 12 and14 inserts all necessary interconnection weights simultaneously and inparallel.

Light passed by holographic array means 12 forms a predetermined patternon a spatial light modulator (SLM) 20 which is disposed in the opticalpath therefrom. Spatial light modulator 20 is of the reflective typeincluding a read side 22 and a write side 24. As apparent from FIG. 1,the interconnection-weight encoded light from holographic array means 12provides the interconnection pattern on read side 22 of the SLM 20 whilea beamsplitter 26 applies optical input signals to its write side 22.SLM 20 modulates the encoded light incident on its read face 22 inaccordance with the optical input signals and reflects the modulatedlight to beamsplitter 28. Meanwhile, the second holographic array means14 encodes laser light from its source 18 with a second set ofinterconnection weights and directs the N⁴ encoded beams to a second SLM30. Beamsplitter 28 directs the modulated light reflected by read side22 of SLM 20 onto the write side 24 of the second SLM 30 as the inputtherefore. In this way, input applied to the write face 24 of SLM 20 isinterconnected to the detecting or write face 24 of SLM 30 by theencoded beams projected by holographic array means 12. SLM 30 thenmodulates the pattern of light incident thereon from holographic arraymeans 14 according to the modulated optical signals applied to its writeside from SLM 20.

Geometrical relay optics 32 directs the modulated light reflected fromthe read side 22 of SLM 30 to the write side 24 of SLM 20 as the newinput for system 10. Accordingly, the encoded beams projected byholographic array means 14 interconnect the write side 24 of SLM 30 tothe write side of SLM 20. It is contemplated that after a certain time,the original optical input to beamsplitter 26 is terminated so that themodulated signals from SLM 30 become the only input signals to the SLM20. At the same time that the modulated signals from SLM 30 are appliedto SLM 20, relay optics 32 provides these modulated optical signals asthe system output.

System 10 according to the present invention can be analyzed asperforming four distinct steps. The first of these steps is a matrixmultiplication

    A.sub.1 X=Y

where A₁ is an N² ×N² matrix and X is a N² ×1 column vector describingthe input to the system 10. The elements of A₁ are considered to be thepredetermined interconnection weights injected into system 10. It willbe recognized by one of ordinary skill in the art that the system 10could be operated as a Hopfield neural network such that when system isqueried by an input vector X, it will provide, as output, a vector whichis most nearly identical to the input vector from the vectors stored inmatrix A. Similarly, where system 10 is arranged as a bidirectionalassociative memory (BAM), it will produce the closest associated vectorsX_(out) and Y_(out) when queried by an input vector X. The second stepcomprises carrying out a nonlinear transformation on Y to generate a newvector Y'. Hereinafter, this step will be described by the relation:

    NL(Y)=Y'

In the third step, a second matrix multiplication is carried out on Y',to obtain Y", namely

    A.sub.2 Y'=Y"

Finally, a second nonlinear transformation represented by

    NL(Y")=X'

is performed to complete one cycle of operation. In each subsequentcycle, the output vector X' can be applied as a new input vector X untilthe system settles on an output which is most nearly associated with theoriginal input vector.

Embodiment With Polarizing, Reflective Spatial Light Modulators

As mentioned in the foregoing, each holographic array means 12, 14 ofholographic means 11 projects up to N⁴ individual beams onto the readside 22 of the SLM 20, 30 in its the optical path. Each individual beamprovides an interconnection in system 10. To accomplish the desiredinterconnections, each individual beam is encoded to have one or moreparameters representing the particular weight accorded to theinterconnection made by that beam. Since all of the N⁴ individual beamsprojected by holographic array means 12, for example, are passed as aresult of exposing the holographic array means to plane waves from itsrespective source 16, all the interconnections made and weighted byholographic array means 12 are projected into system 10 simultaneouslyand in parallel.

Holographic array means 12, 14 are depicted in detail in FIG. 3. Eachholographic array means 12, 14 is of the page-oriented type whichcomprises N² individual holograms 50 arranged in an N×N, spatiallylocalized array. Holograms 50 are affixed in a transparent substrate 52,such as a glass slide. As is known by those of ordinary skill in theart, the individual holograms 50 can be formed from a thermoplasticmaterial or a photorefractive material. Holograms 50 could be recordedpermanently, or alternatively, they could be of the erasable type.Depending upon whether holograms 50 are fixed or erasable, theinterconnections made thereby could be fully in parallel or "partiallyin parallel and partially time sequential". That is, if the holograms 50are permanently recorded, all the interconnections are made and weightedpermanently and therefore system 10 is referred to as fully in parallel.On the other hand, if the holograms 50 are erasable, the interconnectionweights can be changed during operation of system 10 and the system isreferred to as partially in parallel and partially time sequentialbecause the interconnection weights can be changed in time.

As compared to volume holograms used in prior art optical systems, thespatially localized holograms 50 have such a relatively small thicknessas compared to their length and width that they can be considered asapproximately two dimensional. It is contemplated that the hologramscould have a thickness of, for example, 20 μm, and a width of about 1 mmon a side. Accordingly, holographic means 11 can provide for 10¹² ormore interconnections in system 10. Due to the relativetwo-dimensionality of holograms 50, system 10 is not limited by lesseneddynamic range, intermodulation noise and interconnection degeneracy when10¹² or more interconnections are achieved in the system. Likewisesystem 10 avoids the cross talk problems arising in electrical deviceswhen N approaches such a large number.

Each hologram 50 is formed to pass up to N² individual light beams whenit is illuminated by laser light. The holograms 50 of holographic arraymeans 12 are written so as to inherently project the interconnectionweight encoded beams onto defined areas or pixels on the read side 22 ofreflective SLM 20 whereby SLM 20 will reflect the beams to illuminatecorresponding defined pixels on the write side 24 of SLM 30. Theholograms 50 of holographic array means 14 likewise inherently focus N⁴individual beams onto the read face 22 of SLM 30 which similarlyreflects the beams to relay optics 32 that serves to guide the reflectedbeams to predetermined areas on write face 24 of SLM 20. In this way,holograms 50 of holographic array means 12 and 14 are written tosimultaneously interconnect the data input or the write faces 24 of thetwo SLMs 20 and 30 to each other. All of these interconnections are madeoptically and in parallel. Holographic means 12 and 14 thusautomatically provide alignment for the optics in system 10.

As also mentioned briefly in the Overview, it is convenient to think ofthe N⁴ individual beams introduced by holographic array means 12 asrepresenting the elements of an N² ×N² matrix A₁. The elements of matrixA₁ designate the desired interconnection weights for operation of system10. According to the preferred embodiments, the desired magnitude or"strength" of each interconnection is represented by the intensity ofits respective projected beam.

Further, according to the preferred embodiments, the N² light beamsproduced by each hologram 50 are directed to a common location or pixelof the reflecting or read side 22 of SLM 20. In other words, the ithhologram, 50-i projects N² beams to a same pixel, referred to as P₁, ofSLM 20. In this way, the N² holograms 50 of holographic means 12 form N²resolvable patterns on N² different pixels P₁ -P_(N) ² on the read side22 of SLM 20. The active areas on SLM 20, and SLM 30, are contemplatedto be about 10 cm square at about 10 line pairs per mm.

Each of reflective SLMs 20, 30 in system 10 performs a modulationfunction, a detection function and a nonlinear operation. The modulationfunction of SLMs 20, 30 will be considered first. As understood fromFIG. 2, SLM 20 is positioned in the optical path provided by holographicarray means 12 to reflect each pattern of N² beams incident on eachpixel, P₁, of its read face 22, through beamsplitter 28, tocorresponding pixels referred to as P₁ ' through P_(n) ² ' on the writeface 24 of SLM 30. It is also understood that each pixel on the readside 22 of SLM 20 likewise has a corresponding pixel, P₁ ', on its ownwrite side 24. It is onto these corresponding pixels, P₁ ' through P_(N)² ' on the write side 24 of SLM 20 that the input to system 10, in theform of the input vector X is provided through beamsplitter 26. Inputvector X is thought of as a column vector in the form ##EQU1## Whenoptical input signals representative of the vector X are applied to itswrite side 24, SLM 20 responds by modulating the N⁴ interconnectionweight encoded beams projected onto its read side 22 by holographicarray means 12. In the preferred embodiments, this modulation is carriedout by polarization. Depending upon the value of a particular element x₁of X represented by an input beam incident on one of its write sidepixels, P₁ ', SLM 20 polarizes the N² individual beams reflected by itscorresponding read side pixel P₁. Each group of N² beams projected fromeach hologram 50-i thus are polarized according to information carriedby an input beam incident on the write side pixel P₁ ' of SLM 20, andthen reflected by the corresponding read side pixel, P₁ of SLM 20 tobeamsplitter 28. Alternatively, it is also contemplated that the SLMs20, 30 incorporated into system 10 could modulate the interconnectionweight encoded beams in other ways as well. For example, the SLMs couldeffect amplitude modulation or phase modulation. Detection would becarried out accordingly.

In the preferred system 10, beamsplitter 28 analyzes the reflected,polarized beams from SLM 20. Beamsplitter 28 passes light projected byholograms 50 to the read side of SLM 20 without affecting thepolarization of the beams. On the other hand, beamsplitter 28 acts as apolarizer for light entering from the opposite direction, namely lightreflected therethrough from SLM 20. Thus, beamsplitter 28 cooperates asthe analyzer for polarizing reflective face 22 of SLM 20 to modulate theintensity of the N⁴ interconnection-weight encoded beams directed to thewrite side 24 of SLM 30.

FIGS. 4 and 5 are useful in illustrating the modulation performed bypolarizing, reflective SLM 20 and analyzing beamsplitter 28 in a greatlysimplified example where, for purposes of explanation, N is set to equal2. FIG. 4 depicts one of various ways in which the read face 22 of SLM20 may be partitioned to receive the encoded beams projected byholograms 50-1, 50-2, 50-3, and 50-4 as shown. Accordinly, the elementsof the 4×4 matrix A₁, where ##EQU2## are projected onto read face 22.Each row of four partitions 52 on the read face 22 defines one of theread face pixels P₁, P₂, P₃ and P₄ respectively as indicated. Pixels P₁,P₂, P₃ and P₄ correspond with write face pixels P₁ ', P₂ ', P₃ ' and P₄' (not shown in FIG. 4 as they face the plane of the paper) whichreceive light indicative of the elements x₁, x₂, x₃, and x₄ of inputvector X. SLM 20 polarizes the four illustrated beams representative ofelements a₁₁, a₂₁, a₃₁, a₄₁ according to the value of element x₁ of Xand so on according to the value of each element of X. When beamsplitter28 analyzes the four distributions of four beams representative of thecolumns of A₁, this can be thought of as performing the portion of themultiplication of A₁ X=Y which comprises multiplying the four rows ofthe column vector X by the four rows of matrix A₁. Beamsplitter 28analyzes and directs the beams reflected by SLM 20 to write face 24 ofSLM 30. FIG. 5 depicts the partitioning of write face 24 of SLM 30 todelineate the pixels P₁ ' , P₂ ', P₃ ' and P₄ ' as columns on the writeface. The modulated light beams transmitted from beamsplitter 28 thenwould be distributed as indicated in FIG. 5. SLM 30 detects theintensity of each distribution of four modulated beams as each elementof A multiplied by a corresponding element of X on pixels P₁ ', P₂ ', P₃' and P₄ ' to arrive at the product Y=A₁ X. To continue with thisexample in detail, the detection function performed by SLM 30 can bethought of as summing the multiplied elements

    a.sub.11 X.sub.1 +a.sub.21 X.sub.2 +a.sub.31 X.sub.3 +a.sub.41 X.sub.4 =y.sub.1

    a.sub.21 X.sub.1 +a.sub.22 X.sub.2 +a.sub.23 X.sub.3 +a.sub.24 X.sub.4 =y.sub.2

    a.sub.31 X.sub.1 +a.sub.32 X.sub.2 +a.sub.33 X.sub.3 +a.sub.34 X.sub.4 =y.sub.3

    a.sub.41 X.sub.1 +a.sub.42 X.sub.2 +a.sub.43 X.sub.3 +a.sub.44 X.sub.4 =y.sub.4

at its write side pixels P₁ ', P₂ ', P₃ ', and P₄ '. By this detectionand summation of the modulated beam intensities SLM 30 carries out therow-wise summation for the matrix multiplication A₁ X=Y.

SLM 30 performs a nonlinear transformation of each detected element, y₁of vector Y. FIG. 6 is a graphical representation of a function G(Y)which describes the response of SLM 30 to each detected y₁. The functionG(Y) is a continuous, monotone increasing, nonlinear function which mapsthe vector Y in N² space to a new vector Y' also in N² space to providea sigmoidal threshold response. SLMs 20 and 30 are controllable to setthe slope or steepness of their sigmoidal response curve G(Y) dependingupon particular applications of system 10. SLM 30 performs thisnonlinear transformation of Y to complete the second processing stepcarried out by system 10. The new vector Y' resulting from the nonlinearmapping becomes the input for the final two processing steps.

Holographic array means 14 and SLM 30 communicate through beamsplitter34 in the same manner as SLM 20 and holographic array means 12.Holographic array means 14 likewise projects N⁴ beams forinterconnecting SLM 30 back to SLM 20 to complete system 10. Accordingto the preferred embodiments, the N⁴ beams projected by holographicarray means 14 are encoded by intensity to provide the magnitudes of asecond set of interconnection weights. Each hologram 50 of holographicarray means 14 likewise projects N² beams to a pixel of reflecting face22, corresponding to each column of a second matrix A₂.

Step 3 of the processing by system 10 comprises a second matrixmultiplication given by

    A.sub.2 Y'=Y".

Matrix A₂, introduced by holographic array means 14, can be chosen tohave different elements depending upon how system 10 is desired tooperate. This operation likewise is carried out in parallel by thecooperation of SLM 30, holographic array means 14, polarizingbeamsplitter 34, and the write/detection side 24 of SLM 20 in the sameway as described for SLM 20, holographic array means 12, beamsplitter28, and write side 24 of SLM 30. That is, encoded light from holographicarray means 14 is projected onto the read face 22 of SLM 30 andpolarized thereby in response to the vector Y'. Analysis of the N⁴polarized beams reflected by read side 22 at beamsplitter 34 therebyprovides modulated beams representative of the multiplication of each y'by the ith row of matrix A₂. Each of the resulting N⁴ modulated lightbeams is directed to assigned pixels on the write side of SLM 20 asinput therefore by geometrical relay optics 32. SLM 20 likewise detectsthe intensity of the associated beams to perform the row-wise summationand thereby complete the matrix multiplication

    A.sub.2 Y'=Y".

The SLM 20 similarly carries out a second nonlinear transformation ofvector Y" according to a nonlinear threshold function of the form G(Y)in FIG. 6 to obtain the vector X' which becomes the new input for system10.

Rely optics 32 likewise are conventional and any combination thereof fordirecting the modulated beams reflected from read side 22 of SLM 30 towrite side 24 of SLM 20 will suffice. In the embodiment of FIG. 2, relyoptics 32 is seen to comprise a beamsplitter 36 which simultaneouslyprovides the optical signals analyzed by beamsplitter 34 as output forsystem 10 and which directs the analyzed signals to a lens and mirrorarrangement 38. Beamsplitters 26 and 36 are not required to performanalyzing function and therefore do not need to be of the polarizingtype. In arrangement 38, a first lens 40 focuses the analyzed signalsfor reflection by mirror 42. A second lens 44 likewise focuses thereflected signals onto a second mirror 46 which in turn directs theanalyzed signals through beamsplitter 26 as the new input to SLM 20. Inthe embodiment of FIG. 2, the relying is image wise. However, it iscontemplated that, if desired, the relayed pattern could be transformedby the rely optics. It also is contemplated that other arrangements oflenses and mirrors or still other arrangements including prisms could beused for directing modulated light from SLM 30 onto the write side 24 ofSLM 20. Alternatively, the rely optics could be provided in the form ofan arrangement of optical fibers.

Operation Modes

Preferred system 10 provides for several modes of operation dependingupon the desired form of the input vector X and the elements of matricesA₁ and A₂. As mentioned previously, system 10 can be arranged to operateas either a Hopfield neural network or a bidirectional associativememory (BAM) depending upon how the interconnection weights in thesystem are encoded. In particular, operation as a Hopfield neuralnetwork or a BAM depends upon the interconnection weight encoding byholographic array means 14 which projects the elements of A₂. If A₂ isthe N² ×N² identity matrix I_(n) ² where ##EQU3## the system behaves asa Hopfield network. Accordingly, light from holographic array means 14which is reflected by read side 22 of SLM 30 is modulated to simplytransmit the vector resulting from the nonlinear transformationperformed by SLM 30. Thus, in operation as a Hopfield network, the thirdand fourth steps performed during processing by system 10 do not changethe vector Y' obtained as a result of the nonlinear operation carriedout by SLM 30. Rather the vector Y' is re-applied to the write side ofSLM 20 as the new input. When operated as a Hopfield network, the stepsperformed by system 10 therefore can be summarized by the following:

(1) AX=X'

(2) NL(X)=X"

(3) I_(n) ² X"=X"

(4) NL(X")=X"

whereby X" is applied to SLM 20 as input for the next iteration.

Alternatively, if A₂ is encoded as the transpose A^(T) of the Matrix A₁projected by holographic means 12, system operates as a BAM. Thisoperation is summarized by the steps:

(1) AX=Y

(2) NL(Y)=Y'

(3) A^(T) Y'=Y"

(4) NL(Y")=X'

In this mode, system 10 will produce the closest paired vectors X_(out)and Y_(out) associated with the interrogating input vector X.

Independent of whether system 10 is operated as a Hopfield network or aBAM, the system can be operated in a "unipolar" mode wherein all of theelements of the paired vectors X and Y and all of the elements ofmatrices A₁ and A₂ are positive or zero. This unipolar mode isconsidered to be analog operation and accordingly the elements of X, Y,A₁ and A₂ can have any positive value. When system 10 is operated in aunipolar mode, SLM 20 responds to the elements of initial input vector Xby polarizing their associated distributions of interconnection-weightencoded beams such that when the associated beams are analyzed atbeamsplitter 28, analog intensity values are obtained. The intensitiesof the N² beams defining each component y₁ of Y are then summed bydetecting face 24 of SLM 30 as previously described. SLM 30 responds tothe transformed vector Y' to likewise polarize the encoded beamsprojected by holographic array means 14 for analysis at beamsplitter 34.The intensities of associated beams are then likewise detected andsummed at the write or detecting face 24 of SLM 20.

Alternatively, system 10 can be operated in a "bipolar" mode wherein allof the elements vectors of X and Y are defined to have one of twopossible values, for example 0 or 1, or, for example -1 or +1. In thebipolar mode these two different values easily can be represented by twoorthogonal polarization states. The elements of matrices A₁ and A₂ canhave any value in the bipolar mode.

As is understood, system 10 may be operated asynchronously andcontinuously in time. Alternatively, system 10 can be operatedsynchronously according to clock pulses. This is commonly referred to inthe art as "systolic" operation.

Normalization

In order to simplify the detection process in the preferred system 10according to the present invention, it is contemplated to make the inputand output vectors bipolar and to normalize the elements of matrices A₁and A₂. It will be assumed for the following discussion concerningnormalization that system 10 is operated as a BAM, in a bipolar mode.According to this mode, the elements of the input and output vectors Xand Y are provided according to two orthogonal polarization states tohave the two possible values of -1 or +1. These two polarization stateswill hereinafter be denoted as -1 and +1.

While the vector elements attain only the values +1 and -1, the elementsof the matrices A and A^(T) can be any positive or negative numberrepresenting the desired interconnection weights. The beams projected byholographic means 12 and 14 are polarized to either of the twoorthogonal polarization states to encode the sign of the matrixelements. The intensity of each projected beam encodes the magnitude ofthe respective matrix element. Mathematically, the matrix multiplicationrequires only that the elements of the matrices be multiplied by +1 or-1. To carry out the multiplication optically requires no more thanprogramming SLM 20 according to the input vector to reflect eachinterconnection-weight encoded beam either with or without changing itspolarization state.

Simple reflection without changing the polarization state corresponds tomultiplication by unity. Rotation of the polarization state of theencoded beam by 90 degrees represents multiplication by -1.

After multiplication of the matrix elements, detection of thepolarization states and the intensity of the individual beams likewiseare made to determine the elements of Y such that

    y.sub.1 =Σa.sub.1j X.sub.j                           (1)

where a_(1j) is an element of matrix A. Each element of Y receives onecontribution from each element of X. Each element of Y likewise willhave a distinct polarization state indicative of its sign and anintensity indicative of its magnitude. The nonlinear operationthereafter will set the magnitude of each y₁ to +1 or -1. Thus,detection of each y₁ is carried out by determining the dominantpolarization state of each y₁. This detection operation can bemathematically described as ##EQU4## where the symbol "(p)" denotespositive contributions to each y₁ and "(n)" denotes the negativecontributions. If each y₁ is pre-normalized to +1 or -1, the onlyrequired operation is to determine the sign and thus detection issimplified because the magnitudes of the elements are known. It would beadvantageous to perform a first order normalization of each element Y₁in the sense

    Σ|(a.sub.1j x.sub.j)|=1.           (3)

As normalization in this way has been found to be more difficult toperform, it is preferred to normalize the columns of both A and A^(T).Matrices A and A^(T) are considered as a series of columns such that

    A=[a.sub.1, a.sub.2, . . . a.sub.N, . . . a.sub.N.sup.2 ], (4)

    where Σ|a.sub.1j |=M.sub.1.        (5)

If the elements of the vector X which is multiplied by A have values ofonly +1 and -1, the absolute magnitude of any Y₁ also will be M₁. Thepreferred normalization process comprises multiplying each column a₁ ofA and A^(T) by an appropriate scaler factor c₁ such that the sum of theabsolute magnitudes of the elements of a₁ is 1. This may be written as

    A=[c.sub.1 a.sub.1, c.sub.2 a.sub.2, . . . c.sub.N a.sub.N, . . . c.sub.N.sup.2 a.sub.N.sup.2 ]                             (6)

    so that

    c.sub.1 Σ|a.sub.1j |=c.sub.2 Σ|a.sub.2j |=. . . c.sub.N Σ|a.sub.Nj |=1                    (7)

where c₁ =1/M₁, and a_(1j) is the jth element of a₁. Therefore, it isnecessary only to detect the magnitude of one of the polarizationstates. Then the whole detection process in the operation

    Y'=AX                                                      (8)

reduces to the determination of the vector components according to##EQU5## From the foregoing it is seem that

    c.sub.n Σ|a.sub.nj |=α.sub.n (10)

where α_(n=) 1 for an ideal detector. It is not unusual that thedetector threshold may vary across the detecting surface of a givenphotodetector device. Then, it may be required to change thenormalization by selecting the constant α_(n) as determined by practicalconsiderations of the detector used. This normalization does not affectthe needed decision of equation (2) and, thus, does not affect systemperformance in any way other than to simplify the optical components inthe system.

Cascading Networks

It is to be understood that any number of optical networks arranged asin preferred system 10 can be made a part of a larger, overall system.For instance, FIG. 7 is a block diagram showing an exemplary cascadingnetwork 100 according to the present invention. Each network 100comprises a number of stages 110; three such stages are shown in FIG. 7.Where each stage comprises a holographic means, a spatial lightmodulator means and an analyzer as discussed in connection with system10, the analyzed-modulated output beams from each upstream stage areapplied to the SLM of its adjacent downstream stage in a feed-forwardarrangement. Indeed as is apparent, casoading network 100 could be apurely feed-forward arrangement with no feed back as indicated by thesolid ray line 112 in FIG. 7. Alternatively, as indicated by the brokenray line 114, network 100 could be provided with feedback from, forexample, its rightmost or terminal stage to its leftmost or initialstage.

Embodiment With Transmissive Spatial Light Modulators

FIG. 8 shows a system 200 which is arranged with transmissive ratherthan reflective spatial light modulators. System 200 comprises twodistinct optical paths 201A, 201B which are placed in communication byelectronic elements. Each optical path comprises a holographic arraymeans 212, 214 a first or entrance polarizer 216, 218, a transmissiveSLM 220, 222, a second or exit polarizer 224, 226, and a photodetectormeans 228, 230. An electronic driver means 232, 234 controls thetransmissivity of each of SLMs 220 and 222.

Illumination of holographic array means 212 by laser light source 236likewise projects N⁴ interconnection weight encoded beams into thesystem 200. Polarizer 216 polarizes the encoded light beams before theyilluminate assigned pixels of transmissive SLM 220. Input to system 200again can be thought of as an input vector X is programmed into system100 by driver 232. According to the embodiment of FIG. 8, vector X whichis provided in the form of electrical signals. Driver 232 could comprisea computer or the like for controlling its associated SLM 220 tomodulate and transmit beams representative of the column-wisemultiplication of A₁ X=Y. An analyzing polarizer 224 is provided in theoptical path between SLM 220 and its respective photodetector 228. Eachpattern of N² modulated beams directed onto photodetector 228 throughanalyzer 224 likewise corresponds to a component Y₁ of Y.

Photodetector means 228 receives the modulated light beams transmittedthrough SLM 220 and analyzing polarizer 224 likewise to detect theirintensities and thereby perform the matrix multiplication

    A.sub.1 X=Y.

Photodetector means 228 is programmed to provide electrical outputsignals according to the nonlinear function G(Y) illustrated in FIG. 6in response to the summed intensities of the detected optical signals.Accordingly, photodetector means 228 performs the nonlineartransformation NL(Y)=Y' and applies electrical signals indicative of Y'to the driver 234 which controls SLM 222. Driver 234 in turn programsSLM 222 to modulate the encoded beams projected by its associatedholographic array means 214 according to the signals indicative of Y'.In this way, the second matrix muliplication A₂ Y'=Y" is performed whenthe modulated signals transmitted through SLM 222 and analyzingpolarizer 226 are detected by photodetector means 230. Photodetectormeans 230 likewise performs the nonlinear thresholding operationNL(Y")=X'. The resulting vector X' is provided as output for system 200and is fed back to driver 232 and its associated SLM 220 as the nextinput to system 200. System 200 thus is seen to operate in the samemanner as system 10 of FIG. 2. System 200 likewise can be operatedcontinuously and asynchronously.

One of ordinary skill in the art appreciates that different combinationsof reflective and transmissive spatial light modulators and thatdifferent photodetection means may be used for implementation of thesimulating system according to the present invention. It is recognizedthat the spatial light modulating means in the system of the presentinvention could be either optically or electrically addressed. Furthersuch spatial light modulators could be of the polarization-modulation,amplitude-modulation, or phase-modulation type. Detection devices wouldbe provided accordingly to evaluate any given parameter of the modulatedindependent beams and perform the necessary nonlinear operations on thedetected data.

As for the photodetection means, FIG. 9 illustrates a photodetectormeans 300 which has a summing-detecting surface 302, a plurality oflight emitting diodes 304 (LEDS) and a driver 306 for the LEDS.According to this arrangement, each LED is made to generate light inaccordance with the result of the nonlinear transformation performed bythe photodetector means 300. It is further contemplated that, withrespect to FIG. 8, photodetector means 230 could comprise a variety ofdifferent photoresponsive devices. For example, the photodetector means228 and 230 could comprise charge coupled devices (CCD) or chargeinjection devices (CID). These devices could be provided in spatiallydiscreet or spatially continuous arrays as understood in the art.Further still, it is contemplated that the photodetector means 228 and230 of FIG. 8 could be replaced by a television detection systemincluding vidicons (not shown). Any such implementation of thephotodetection means can be provided to have the sigmoidal responsecurve of FIG. 6 for carrying out the prescribed summation-detection andnonlinear transformation. Moreover, these aforementioned electricaldevices, as well as many optical detecting devices including spatiallight modulators can be adjusted to adjust their response curves toprovide a clipping response as illustrated in FIG. 10. Similarly, manysuch devices can be made to provide a level-restoring response forsetting their outputs to 0 to 1 according to binary operations.

Embodiment with Bistable Detector

In FIG. 11 there is shown an embodiment of a system 300 of the presentinvention wherein an alternative optical detecting means 302 issubstituted for the downstream spatial light modulator. Opticaldetecting means 302 comprises a polarizer 304 and an optical device 306located to receive light passed by polarizer 304 from a reflective SLM308. Polarizer 304 analyzes the reflected beams from SLM 308. Thelight-detection surface 310 of device 306 is thus illuminated by the N⁴modulated beams reflected by the read side 312 of SLM 308 and is alsoilluminated by a pattern of collimated laser beams, which hereinafter,will be referred to as a pattern of holding beams 314. In the embodimentof FIG. 11, device 306 is of the spatially continuous type wherein itsdetecting surface 310 is a continuous plane. Alternatively, it isrecognized that device 306 may be made up of a plurality of discretedevices arranged to form a composite detection surface.

Optical device 306 responds to the individual modulated light beams fromSLM 308 and holding beams 314 by either passing each superimposedmodulated beam and holding beam to relay optics 316 or by obstructingthe superimposed beams. In preferred system 300, device is a bistabledevice having a nonlinear response curve which is shown in FIG. 12. Thisresponse provides two regions of stability between points s₁ and s₂.When the combined light intensity from the holding beam 314 and theindividual modulated beams from SLM 308 exceed a threshold level S₀,device 306 passes the superimposed beams therethrough as a resultantbeam to the downstream geometrical relay optics 316. Each resultant beamcorresponds to a Y₁ having the value +1. If the combined signal beam andholding beam intensity do not exceed threshold S₀, then the element Y₁will be registered as a 0. In the preferred embodiment, bistable devicehas a threshold set at α₁ /2. Due to the presence of the holding beams314, the output information beams are amplified. The amplification orgain provided by bistable device 306 is dependent upon the intensity ofthe holding beams. Like the SLMs which perform the detection functionfor systems 10 and 200, bistable device can be set to effect clipping orlevel restoring.

As apparent from FIG. 11, SLM 308 is also of the reflective type whichmodulates the encoded beams projected from holographic means 318 bypolarization. In operation, the input vector X preferably is introducedinto system 300 through beamsplitter 320 to SLM 308 as a binary vectorhaving elements comprising only 0 or 1. This pattern serves as the writebeam incident on the write side 322 of SLM 308. If an element x₁(incident on a pixel of read side 322) of input vector X has the value1, SLM 308 will reflect the associated interconnection weight encodedlight from holographic means 318 in its orginal polarization torepresent multiplication by +1. Unaddressed, i.e. unilluminated, pixelson the write side 322 will signal SLM 308 to rotate the polarization ofthe associated encoded beam by 90 degrees to effect multiplication by -1as the beams are reflected.

Holographic means 318 comprises two complete arrays of N² page-orientedholograms of the type discussed in connection with FIG. 3. One arraystores the matrix A and the other A^(T). After illumination by lasersource 326, holographic means 318 projects the images of both A andA^(T) onto SLM 308. The sign of each matrix element is encoded by one oftwo orthogonal polarization states. Beam intensity encodes the magnitudeof each element. The image, carrying the results of the vector-matrixmultiplication, is reflected back through beamsplitter 324 and focussedonto detecting means 302.

So arranged with holographic means 318 providing both matrices A andA^(T) preferred system 30 operates as a BAM. The light patternrepresenting input vector X, represented by the solid ray line in FIG.11, is applied to a designated portion of the write side 322 of SLM 308which write side portion corresponds to an associated portion of itsread side 312 receiving interconnection weight encoded beams indicativeof the elements of matrix A. The intensities and polarization states ofthe projected beams from holographic means 318 represent the magnitudeand sign of the interconnection weights represented thereby. Theseencoded beams are polarization modulated by SLM 308 according to theinput vector 304. Analysis of the reflected modulated beams is performedat polarizer 304. Bistable device 306 thereafter automatically detectsthe beams passed by analyzing polarizer 304 and performs the matrixmultiplication step

    AX=Y

and the nonlinear operation

    NL(Y)=Y'

on the detected beams and the incident holding beams 314 in accordancewith the response curve shown in FIG. 12. Beam patterns representingvectors Y and Y' also are understood as indicated by the solid ray linesin FIG. 11. Vector Y' is thus derived from the modulated beams and theholding beams, and it likewise provided in binary form to be routed backto SLM 308 for further iterations. At the same time, it is also madeavailable as an output. In the second iteration, Y' is multiplied by thematrix A^(T) as indicated by the dot-dash ray-line to complete thesecond matrix multiplication step

    A.sup.T Y'=Y".

As seen from FIG. 11, this alternate multiplication by A and A^(T) isachieved by focusing the beam patterns indicative of X, A, A^(T), Y, Y',and Y" onto the two designated locations or portions on SLM and thebistable device 306. The formation of holographic means 318automatically provides for the simultaneous projection of the images formultiplication by A and A^(T). Accordingly, when vector Y' is relayedback to the corresponding location on SLM 308, the SLM performs thesecond matrix multiplication

    A.sup.T Y'=Y"

whereafter detecting means 302 analyzes the reflected beams and performsthe second nonlinear transformation on Y" according to

    NL(Y")=X'

to provide the new input vector X'. Operation thus continuesasynchronously or synchronously as desired.

It is noted that vectors X and Y need not use up all of the availablepixels of SLM 308. Rather, as understood by those of ordinary skill inthe art it is possible to partition the inputs and outputs in a fashionsuch as that disclosed in "Architectures for Optoelectronic Analogs ofSelf-organizing, Neural Networks", Optics Letters, 12, 6, (1989) toachieve simulation of multilayer systems and/or to provide variousfeedback arrangements. Various relay optics arrangements would beselected accordingly. In view of the large number of vector elementsavailable in this simulated BAM, such partitioning can be handled rathereasily. The actual partitioning is arbitrary, and projection of thematrixes can be adjusted accordingly.

An alternative to the holding beams in the embodiment of FIG. 11 wouldbe to adjust the intensities of the interconnection weight encodedbeams. Holographic means 318 could be made to project encoded beamshaving sufficient intensities to obviate the holding beams. Further, itis understood that the intensity of the interconnection weight encodedbeams could be adjusted to accommodate for threshold variations in thedetecting surface 310.

It is to be understood that there are various changes and modificationsto the system disclosed herein which changes and/or modifications may bemade by one of ordinary skill in the art, but such skill would result ina system well within the scope of the invention as set forth in theclaims.

We claim:
 1. An apparatus for simulating a highly interconnected neuralnetwork, said apparatus comprising:means for applying input signals;page-oriented holographic means for providing first optical signalsindicative of first predetermined interconnection weights; first spatiallight modulating means responsive to input signals from saidinput-signal applying means for modulating first optical signalsprovided by said holographic means to provide first modulated opticalsignals and; means for detecting first modulated optical signalsprovided by said spatial light modulating means and for performing anonlinear transformation of a parameter of said first modulated opticalsignals.
 2. An apparatus as claimed in claim 1, wherein said holographicmeans comprises a first holographic array means including a plurality ofholograms and a substrate for spatially arranging said holograms in anarray.
 3. An apparatus as claimed in claim 2, wherein said means fordetecting and transforming comprises a second spatial light modulatingmeans.
 4. An apparatus as claimed in claim 3, wherein said page-orientedholographic means comprises a second holographic array means includingholograms for providing second optical signals indicative of secondpredetermined interconnection weights, and wherein said parameter ofsaid first modulated signals is the intensity thereof;said secondspatial light modulating means modulating second optical signalsprovided by said second holographic arrays means to provide secondmodulated optical signals as a nonlinear function of the intensity offirst modulated signals provided by said first spatial light modulatingmeans.
 5. An apparatus as claimed in claim 4, wherein one of saidspatial light modulating means comprises a reflective spatial lightmodulator.
 6. An apparatus as claimed in claim 4, wherein one of saidspatial light modulating means comprises a transmissive spatial lightmodulator.
 7. An apparatus as claimed in claim 4, wherein said first andsecond modulating means each comprise spatial light modulators andwherein said applying means comprises geometrical optical means forproviding an optical path from said second spatial light modulator tosaid first spatial light modulator.
 8. An apparatus as claimed in claim4, wherein said holograms of at least one of said holographic arraymeans are erasable whereby said interconnection weights can be varied.9. An apparatus as claimed in claim 2, wherein said detecting andtransforming means comprises photodetector means for providingelectrical signals representative of a nonlinear transformation of saidparameter.
 10. An optical processing method comprising the stepsof:illuminating a page-oriented holographic means so that it projectsweighted light beams which are each encoded to represent aninterconnection weight onto a spatial light modulator means; applyinginput signals to said spatial light modulator means to cause saidmodulator means to modulate said weighted light beams projected thereonto provide modulated light beams; detecting a parameter of saidmodulated light beams; performing a nonlinear transformation of saidparameter of said modulated beams to provide a nonlinearly transformedparameter, and providing detection signals indicative of saidnonlinearly transformed parameter.
 11. An optical processing method asclaimed in claim 10, further comprising the step of:providing saiddetection signals to said spatial light modulator means as the inputsignals therefor.
 12. An optical processing method as claimed in claim10, whereinsaid page-oriented holographic means projects second weightedlight beams which are each encoded to represent an interconnectionweight onto said spatial light modulator means; and wherein said methodfurther comprises the steps of applying said detection signals to saidspatial light modulator means to cause said modulator means to modulatesaid second weighted light beams to provide second modulated lightbeams; detecting a parameter of said second modulated light beams; andperforming a nonlinear transformation of said parameter of said secondmodulated light beams and providing output signals indicative thereof.13. An optical processing method as claimed in claim 13 wherein saidholographic means comprises a first array of spatially-localized planarholograms for providing said first weighed light beams and a secondarray spatially localized planar holograms for providing said secondweighted light beams and wherein said spatial light modulating meanscomprises two spatial light modulators.
 14. An optical processing methodas claimed in claim 12, wherein the matrix operation steps of:(1) A₁ X=Y(2) NL(Y)=Y' (3) A₂ Y'=Y" (4) NL(Y")=X'are carried out and wherein: saidinput signals represent an input vector having N² elements x_(i) ; saidprojected weighted light beams represent elements of the N² ×N² matrixA₁ ; said detection signals represent a vector Y' having N² elementsy_(i) ; said second projected weighted light beams represent elements ofthe N² ×N² matrix A₂ ; and said output signals represent a vector X'having N² elements.